Polyethylene naphthalate fibers exhibit high tenacity, high modulus and excellent dimensional stability, and is now being applied widely to the field of industrial materials including a tire cord and a rubber reinforcing material for a driving belt and the like. In particular, they are superior to polyethylene terephthalate fibers having been conventionally used since they attain both high strength and dimensional stability, and are strongly expected as a substitute thereof. Polyethylene naphthalate fibers contain molecules that are rigid and liable to align in the fiber axis, and therefore are superior to the conventional polyethylene terephthalate fibers since they attain both high strength and dimensional stability.
For maximizing the characteristics thereof, Patent Document 1, for example, discloses polyethylene naphthalate fibers that are excellent in strength and hot air shrinkage by high-speed spinning polyethylene naphthalate fibers. However, there is a problem that the fibers exhibit high hot air shrinkage when they have high strength, and the strength thereof is decreased when the hot air shrinkage is suppressed, thereby failing to attain a satisfactory level.
Patent Document 2 discloses polyethylene naphthalate fibers that have a tenacity of 7.0 g/de (ca. 6 cN/dtex) or more while maintaining the hot air shrinkage to the same level, by providing a spinning chimney heated to 390° C. immediately beneath the melt-spinning die(spinneret) to perform high-speed spinning and hot drawing. However, the fibers that are obtained in the best example still have an insufficient tenacity of 8.0 g/de (ca. 6.8 cN/dtex), and thus are not satisfactory as fibers having high strength while maintaining heat resistance and dimensional stability.
As different from Patent Document 2, Patent Document 3 proposes polyethylene naphthalate fibers that have high strength and relatively good heat stability formed in such a manner that an undrawn yarn formed with a drawing speed of 1,000 m/min or less and a low draft of about 60 times is subjected to delayed cooling with a spinning chimney having a length of from 20 to 50 cm and an atmospheric temperature of from 275 to 350° C., and then to stretching at a high draw ratio. Patent Document 4 proposes polyethylene naphthalate fibers that have high strength and excellent dimensional stability formed in such a manner that an undrawn yarn having a low birefringence of from 0.005 to 0.025 is obtained at a spinning draft ratio of from 400 to 900, and is then subjected to multi-stage draw at a total draw ratio of 6.5 or more.
These methods provide improvement of a single property among strength, hot air shrinkage and the like of fibers. However, polyethylene naphthalate fibers obtained by any one of these methods still involve such a problem that they are rigid as compared to conventional polyethylene terephthalate fibers and are inferior in fatigue resistance in a composite material. In particular, they have a problem that the fibers are inferior in durability when they are formed into a composite material, in which the fibers receive repeated load, such as those for reinforcing rubber.
(Patent Document 1) JP-A-62-156312
(Patent Document 2) JP-A-06-184815
(Patent Document 3) JP-A-04-352811
(Patent Document 4) JP-A-2002-339161